The present invention relates to high density random access data storage, and is more especially directed to an optical memory system in which laser light is employed to write and read data via a single-photon process in a confined volume. A key feature of the invention is the use of a photosensitive substance that changes among several states when irradiated with light of known wavelengths. One example of such a substance is a light-sensitive protein called bacteriorhodopsin. This protein is capable of changing in its light absorbing and transmitting properties by illuminating it with certain specific wavelengths of visible light. The protein is also capable of emitting an electric signal indicative of the binary state of an irradiated cell of the confined volume.
A number of attempts have been made to produce optical memories that employ a nonlinear optical process. U.S. Pat. No. 4,485,345 describes a two-photon process within an irradiated volume that exposes a sensitized medium. The medium is a four-level material, and the two-photon process generates a microscopic interference pattern. The gating property of the two-photon photochemistry provides for nondestructive reading. Patterns can be produced at several depths below the surface, and thus a three-dimensional capability is possible. A key feature of the branched-photocycle memory is the use of linear, rather than non-linear, optical activation. The advantages are explored below.
Another two-photon, three-dimensional memory has been proposed based on the use of two distinct photochemical forms of spirobenzopyran. In this approach the storage medium is an unoriented organic chromophore in a polymer matrix. The writing process involves a two-photon induced photochemical change involving heterolytic cleavage, the read process is based on the observation of fluorescence from the merocyanine form, and there are blurting effects from the two-photon induced photochemistry that occurs in adjacent bit cells outside of the irradiated volume. One version of this type of memory device is discussed in U.S. Pat. No. 5,268,862.
Another storage device that employs a volume of field-oriented bacteriorhodopsin in a polymer medium and contained in a mechanically displaceable vessel is described in U.S. Pat. No. 5,253,198 to Robert R. Birge and Deshan S. K. Govender. This memory device involves two lasers oriented along respective orthogonal axes that intersect within the volume. Both lasers are pulsed on for writing at a bit cell, and then a photochemical process cleaning is carried out by actuating the two lasers non-simultaneously. The read cycle involves actuating two lasers, and then discriminating the "1" or "0" state of the interrogated bit cell from the electric signal generated by the medium. This process does have some disadvantages in that it requires mechanical displacement of the volume relative to the read/write lasers, and in that the writing process will inevitably create some conversion of the bacteriorhodopsin in adjacent bit cells.
A number of other U.S. Patents concern themselves with the general field of optical memories, and of these several involve two-photon writing or reading.
U.S. Pat. No. 5,124,944 relates to an optical storage device with a matrix that contains "guest" molecules that serve as electron donors or electron acceptors. The medium stores two bits of data that are separated in the frequency domain. The method of storage involves creating "holes" in the absorption band to correspond to the frequencies of the particular fight components.
U.S. Pat. No. 3,480,918 concerns a three-dimensional optical memory, but employs a light-excitable impurity semiconductor.
U.S. Pat. No. 4,078,229 concerns a three-dimensional optical memory with a two-photon system of intersecting beams. This can employ certain specific media, such as diphenylcyclopentadiene Cr(CO).sub.6 dissolved in methylmethacrylate containing azoisobutyronitrile, and barium acrylate dissolved in gelatin containing p-toluenesulphinic acid.
U.S. Pat. No. 4,479,199 concerns a photon echo optical memory system where an echo cell contains a medium that has at least one photo-excited state.
U.S. Pat. No. 5,223,355 relates to a holography system that uses a bacteriorhodopsin-based medium and in which circularly polarized light is used to record a hologram while linearly polarized light is used to reconstruct the hologram.
U.S. Pat. No. 5,283,777 relates to a two-photon, three-dimensional optic recording medium, and a technique that employs photopolymerization.
U.S. Pat. No. 5,289,407 concerns an optical memory device, such as a write-once, read-many-times memory (WORM), configured as a two-photon, three-dimensional memory. The optical film in the device has several distinct bit planes.
During the past decade, the speed of computer processors has increased between two and three orders of magnitude (i.e., a factor of between a hundred and a thousand), with the largest increase occurring in small desktop workstations. This dramatic increase in processor capability has been unmatched by any corresponding increase in data storage densities, which have increased only by one order of magnitude both in random access memory and in sequential (hard disk) memory technology. Meanwhile, the computer arts have entered an era where a majority of computational algorithms are limited less by computer speed than by on-line data storage capacity, and the cost of a typical scientific workstation is determined in large part by the costs of random access memory (RAM) and disk storage capacity. As the computer arts move into the future, it is clear that the, costs of computer system are becoming more and more memory driven, and this trend will continue until a new memory architecture evolves.
One new architecture that has received some attention during recent years is a technique that employs a two-photon absorption process to store data in three dimensions. Memories based on this architecture read and write information by using orthogonal laser beams to address an irradiated cell, on the order of 200 .mu.m.sup.3, within a much larger volume of a nonlinear photochromic material. A two-photon process is used to initiate the photochemistry, and this process involves the unususal capability of some molecules to capture two photons simultaneously to populate an energy level within the molecule with an energy equal to the sum of the energies of the two photons that are absorbed. Because the probability of two-photon absorption process scales as the square of the intensity, photochemical activation is limited to a first approximation to regions within the irradiated volume. Unfortunately, photochemistry outside of the irradiated volume cannot be eliminated entirely, and this leads to ensuing problems.
The three-dimensional addressing capability derives from the ability to adjust the location of the irradiated volume in three dimensions. Two dimensional optical memories have a storage capacity that is limited to 1/.lambda..sup.2, where .lambda. is the wavelength employed, which yields approximately 10.sup.8 bits/cm.sup.2. In contrast, three-dimensional memories can approach storage densities of 1/.lambda..sup.3, which yields densities of approximately 10.sup.12 bits/cm.sup.3. In principle, a two-photon three-dimensional memory can store roughly three orders of magnitude more information in the same size enclosure relative to a two-dimensional optical disk memory. In practice, optical limitations and issues of reliability lower the above ratio to values closer to 100. Nevertheless, the two-photon approach makes parallel addressing of data possible, which enhances data read/write speeds and system bandwidth.
Although two-photon volumetric memories offer the significant storage density advantages outlined above, there are disadvantages that prompted the development of the optical architecture as it has evolved to this point. First, a two-photon process can only be initiated under conditions of high photon flux. Although the use of sharp focussing and molecules with large two-photon absorptivities makes the two-photon technique viable, the flux requirements remain significant. While these, high flux requirements do not absolutely preclude commercialization, the cost of generating high light flux intensities represents a potential barrier to commercialization. Of equal importance is the tendency of all two-photon architectures to generate unwanted photochemistry outside the targeted irradiated cell or volume. In one approach, cleaning pulses are required for resetting the memory cells outside of the irradiated volume. An alternative is to select wavelengths and intensities that independently have a low probability of initiating two-photon processes. This technique been discussed in D. A. Parthenopoulos and P. M. Rentzepis, 245 Science 843-845 (1989). The intensities of the two beams are adjusted so that the laser with a wavelength with a higher probability of inducing two-photon intensity is the more intense beam. The combination of these two characteristics dramatically decreases the amount of photochemistry outside the irradiated volume. Unfortunately, this technique does not eliminate spurious photochemistry, and a large number of read/write operations can still lead to loss of data integrity. Materials with narrow absorption bands work best with single wavelength photochemical cleaning. Regardless of this, virtually all two-photon volumetric schemes require some refresh or cleaning methods to preserve data integrity.
One remaining two-photon approach is worth a few words of discussion, because of its potential to eliminate the need for cleaning pulses or refresh methods. The architecture for this approach is based on the use of independently-phased or binary-phased laser arrays. The independently-phased array architecture is based on the use of linear or rectangular arrays of diode lasers which can be independently phased by external control. A single location is addressed by adjusting the phase of each of the coherent light sources (lasers) so that at the desired "focus" point, the phases of the individual light sources are identical, i.e., "in phase". The constructive interference point is considered descriptively as the "focus" point. That is, the light is phased, rather than actually focussed, to generate constructive interference at a specific location, and to create destructive interference everywhere else. If a sufficient number of coherent fight sources are present, and their geometry is correctly chosen, locations outside of the irradiated volume will be "irradiated" by light of random (i.e., destructive) phase and no excitation will take place. A single linear array of 200 phased laser diodes can achieve near diffraction-limited performance. In practice, it is not possible to phase lasers perfectly, and simulations including phasing errors suggest that actual devices will require two orthogonal linear arrays each with 256 elements. Alternatively, one can use fixed phase lasers and selectively turn on only those lasers that are properly phased relative to the position of the irradiated volume. This architecture is referred to as a binary phased array, or simply a binary array, and while it requires typically four times as many lasers to accomplish diffraction-limited performance, it is more easily implemented in both hardware and software. Both types of devices are currently under investigation in laboratories in North America, Europe, and Japan. Successful implementation of large scale optical arrays has not yet been demonstrated, and so the commercial viability of this architecture is rather uncertain. Therefore, it is of utmost importance to investigate architectures that resolve the problem of unwanted photochemistry without relying on optical addressing architectures such as phased arrays, which are high-risk and expensive to implement. Accordingly, the techniques that are considered should optimally be implemented either with or without phased arrays, and potentially with either one-photon or two-photon architectures.